Printing machines normally include a printing couple which comprises a number of cylinders and/or rollers. A stack of sheets is stored at a feed end of the machine with a reciprocating suction feed assembly being used to deliver the sheets seriatim to the printing couple. In such printing machines a vacuum pump is coupled to the suction feed to lift the top sheet in the stack and the suction feed is mechanically driven to deliver the lifted sheets to a conveyor structure which carries the sheet to the printing couple. Once the sheet is so delivered, a vacuum release is actuated to release the vacuum at the suction feed to release the sheet.
One of the problems with feeding systems in machines of the character described is the inability to account for different weight stock. Vacuum can be selected to effectively operate for a single given weight of stock. If the stock is heavier, then the suction might be insufficient to lift the sheet. Conversely, if the stock were lighter, then the suction feeder could pick up multiple sheets or the sheets could buckle.
The problem is compounded when stock of different weights is provided in a single stack which is to be printed during a single run. Heretofore, in such a situation it is necessary to select a given level of vacuum and hope for minimal doubles and misfeeding problems, or periodically shut the machine off during a single run to adjust vacuum. Both alternatives are, of course, undesirable.
The present invention is directed to solving one or more of the problems discussed above.